AN IMPLANT

Abstract: An implant intended to be fixed through contact with new grown bone tissue, which implant at least partially consists of a dense material, which at least within a portion (1) of its surface, which surface portion shall form a contact surface for new grown bone tissue, has surface pores (2) which cover 5-40 % of said surface portion (1). Close to at least a substantial fraction of all of said surface pores there is at least one elevation (3) which extends over the implant surface which surrounds the brim (edge) of the pore.

Description: AN IMPLANT TECHNICAL FIELD The present invention generally relates to the field of medical applications, more particularly to the field of implantology, and concerns an implant intended to be fixed through contact with new grown bone tissue, which implant at least partly consists of a dense material having, at least within a portion of its surface, surface pores, which cover 5-40 % of said surface portion which shall constitute a contact surface for new grown bone tissue. BACKGROUND ART When using supporting implants it is important that the implant material has a high strength and that a sufficiently high resistance to shearing forces is developed between the implant and new grown bone tissue; the latter feature being important for the achievement of a good retention. A high strength of implant materials can be achieved by using dense materials having an inherent high strength, such as conventional construction materials, for example stainless steel, cobalt-chromium alloys, titanium and titanium alloys, ceramic materials or polymers or materials with controlled defects, including pores. For the fixation of implants it is known in the art to utilize a topographic surface or pores, wherein retention is achieved through the establishment of a good contact between the implant and new grown bone tissue. In the Swedish patent No. 468 502, certain aspects relating in the first place to the pore size distribution in porous implant materials have been disclosed. Specific and complex pore size distributions in this respect have been found possible to use for depositing bone growth promoting substances and in order to stimulate a good bone ingrowth in larger pores. In a Swedish patent application No. 9200072-S is described how the micro porosity of an implant can be utilized for the deposition of one or more bone growth promoting substances by means of carriers prior to implantation. By filling the pores to a different degree with carriers having a poor solubility, and with appropriate active agents, the formation of pores for bone ingrowth and, for release of active substances can be controlled in order to achieve an optimal ingrowth. BRIEF DISCLOSURE OF THE INVENTION It is an object of the invention to provide an implant material having an improved retention. This is achieved, according to the invention, therein that close to at least a substantial fraction of all of said surface pores there is at least one elevation which extends over the implant surface surrounding the brim of the pore. The elevation or the elevations is/are not bound to any specific topography, but preferably they should have the shape of one or more ridges which partly or completely surround the pore. In order that the desired improvement of the retention shall be achieved, at least any elevation around the pore should have an altitude amounting to at least 1 μm, suitably at least 5 μm, over the surrounding material surface. The height is limited in first place by conditions which have to do with production technology. Normally therefore the maximal altitude is not more than 50 μm, preferably max 20 μm. The elevation or elevations around the surface pores, according to the invention can be produced according to various techniques. In those cases when the implant material at least within the portion in question of the surface of the implant consists of a material having a good ductility, such as most metals (also alloys are included in the concept of metals), the surface pores are suitably produced through thermal etching or through supersonic working. Depending on the curvature of the implant, supersonic working can be chosen, which is to be preferred on comparatively flat regions, or thermal etching for more curved parts. For the latter case, more particularly laser should be used. Preferably there is used a laser of the carbon dioxide type; YAG laser, or Excimer laser. By proper arrangement of the apparatus a desired geometry and pattern can be achieved, and by proper adjustment of the supplied power, the elevations around the pore edge can be created through crater formation at the establishment of the pore in those cases when the material is sufficiently ductile. Which materials that are sufficiently ductile and how the pore formation shall be performed in order to create the said crater formation and hence the desired elevations can be determined through empirical experiments. In those cases when the material within the surface portion in question has so poor ductility that any elevations can not be formed, such as craters, at the establishment of the pore by means of laser or the like, which is normally the case e.g. for ceramics, the desired elevations around the pore instead can be established by removing material in a region around the brim portion of the pore, so that the brim portion will remain as an elevation over the surrounding surface. Generally it can be said that through the invention there is achieved an improved balance between the retention and the strength of the implant material. Through optimized ingrowth of bone tissue in surface pores having beads (elevations) according to the invention, the resistance against shear forces between implant and new grown bone can be increased from about 3-4 MPa for cylinders without surface pores to more than about 8 MPa for cylinders having surface pores. Further characteristic features and aspects of the invention will be apparent from the following embodiments and from the appending claims. BRIEF DESCRIPTION OF DRAWINGS In the following description, reference will be made to the accompanying drawings, in which Fig. 1 A-C illustrate examples of some various patterns which the surface pores can form on the implant surface, Fig. 2 shows some surface pores in Fig. 1 A at a larger scale, Fig. 3 schematically shows a section through a surface pore along a line III-III in Fig. 2, and Fig. 4 shows a section through an embodiment having a theoretically ideal design with inclined surface pores. DETAILED DESCRIPTION OF THE INVENTION In Fig. 1 A a portion of an implant surface 1 has surface pores 2 arranged in a square pattern. Fig. IB shows a portion of an implant surface 1 having surface pores 2 arranged in a hexagonal or rhombic pattern. Fig. 1C shows a portion of an implant surface 1 having surface pores 2 arranged in a spiral pattern. In Fig. 2 there is shown a nuπr r of surface pores 2 having elevations 3 extending more or less continuously around th_ _dge of the pore. The elevations 3 have the form of crater formations, which may arise at the formation of the surface pores through working by means of laser if such parameters as supplied power, time etc. are properly set. Fig. 3 illustrates how a po ?, and the region adjacent to the pore may look according to the invention in a typical ..__e. In the drawing, the diameter of the pore 2 is designated D and its depth is designated d. The maximal height of the elevation or the elevations 3 over the material surface 4 surrounding the edge of the pore 2 is designated h. The ratio between the diameter and the depth of the surface pores, D/d, and the height h of the elevation 3 are important for the retention. Thus D/d should lie within the range OJ-10, preferably in the range 0.5-2, when the mean diameter of the surface pores 2 lies betwee 50-500 μm and the surface pore depth is between 10-400 μm. The height h under these conditions should be at least 1 μm, preferably at least 5 μm, while the maximal height h may be up to 50 μm , preferably max 20 μm. Preferably, the mean diameter of the surface pores 2 should be between 100-200 μm, the surface pore depth should be between 50-200 μm, and the distance between adjacent surface pores should be between 50-200 μm. Fig. 4 shows surface pores 2' having inclined pore walls 5, which in combination with the elevations 3 further enhances the retention. The inclination of the pore walls 5 may amount to about 30° relative to the normal directions of the surface pore openings. EXAMPLES 24 cylinders of a completely dense, i.e. without any pores interior of the surface, C P titanium having a diameter of 2.8 mm and a length of 6 mm were implanted in the femur of rabbits for 4 and 12 weeks, respectively. The rabbits were of type New Zealand White having a weight of about 4 kg. Half of the cylinders had pores, which were thermally etched by means of laser having the following apparatus parameters, wherein surface pores were achieved having a mean diameter of about 150 μm and a depth of the same order. Ridge shaped elevations were formed around the opening of the pores through th laser treatment, the ridges having a maximal height over the surrounding material surface of about 5 μm. The mean distance between the surface pores was about 120 μm. Laser type YAG The laser beam was moved to and fro over the cylinder surface in the axial direction, the cylinder being turned between each change of movement direction Rate of movement: 630 mm/min Turning angle appr 14° Pulse frequency 30 Hz Pulse width 0J3 ms Lamp voltage 600 V Pulse power 0.6 J Beam path aperture 3.0 mm Focal point 4.2 mm over the material surface Prior to sterilization the implant samples were cleaned ultrasonically in HCl-solution for 1 h, whereafter the samples were stored in 1 M HCl-solution for 14 h. The samples finally were washed, firstly in di-ionized water and 70 % alcohol and secondly in destillated water. The samples were sealed in special bags and autoclaved for 20 min. In the femure (a leg) of each rabbit three holes were drilled by means of a special drilling device at a distance of about 10 mm from each other and at a distance from the growth zone in cortical bone under a heavy saline flow in order to promote efficient cooling. A low drilling pressure was applied. The drilling produced holes having appr 0J mm play for the cylinder samples. After the period of implantation, the animals were put to death by an overdose of Mebumal, and were prepared for so called push-out tests and for histological evaluation. A 10 mm long bone section with the implant in the centre thereof was prepared from the femure. The bone sections were cut longitudinally in order to make available the part of the implant facing the bone marrow. The prepared bone was kept in 0.9 % NaCl solution without any fixation. Bone with cylinder was placed in a push-out fixture by means of a dental cement. The maximal force required for loosing the implant from the bone was detected by means of a universal instrument for measurement of strength (Alwetron) having a loading rate of 0.5 mm/min. The shear force between implant and bone was calculated through measured power divided by the present contact surface between bone and implant. The results are given in Table 1. Table 1 - Shear power between implant and bone U = without surface pores M = with surface pores Animal Implant 1 Implant 2 Implant 3 Animal Implant 1 Implant 2 Implant No MPa MPa MPa No MPa MPa MPa 6 weeks 12 weeks 1 U 2.1 M 8.3 M l 1.4 5 U 3.7 M 15.8 M 17.2 2 U 1.6 M 7.9 M 8.9 6 U 4.3 M 14.9 M 20J 3 U 1.9 U 2.3 M 7.7 7 U 3.3 U 4.7 M 18.8 4 U 2.2 U 0.8 M 10J 8 U 4.2 U 3.9 M 16.4

Claims: CLAIMS 1. An implant intended to be fixed through contact with new grown bone tissue, which implant at least partly consists of a dense material having, at least within a portion (1) of its surface, surface pores (2), which cover 5-40 % of said surface portion (1), which shall constitute a contact surface for new grown bone tissue, c h a r a c t e r i z e d in that close to at least a substantial fraction of all of said surface pores there is at least one elevation (3) which extends over the implant surface surrounding the brim (edge) of the pore. 2. An implant according to claim 1, wherein the elevation or the elevations consist of a ridge or ridges completely or partially surrounding the pore. 3. An implant according to claim 1 or 2, wherein the maximal height (h) of the elevation over the material surface (4) amounts to at least 1 μm and maximal 50 μm. 4. An implant according to claim 3, wherein the maximal height of the elevation of the over the implant surface (4) amounts to at least 5 μm but not to more than 20 μm. 5. An implant according to any of claims 1-4, wherein at least that part of the implant surface which comprises the portion (1) having said surface pores consists of a metal, of a polymeric, or of a composite material substantially consisting of a metal or of a polymer. 6. An implant according to any of claims 1-4, wherein at least that part of the implant which comprises the portion (1) having said surface pores consists of a ceramic or of a composite material substantially consisting of a ceramic material. 7. An implant according to any of claims 1-6, wherein the surface pores as well as the elevations which completely or partly surround the surface pores are produced through laser treatment of the implant material.

Inventors: HARRYSSON RALPH, HERMANSSON LEIF, JOHANSSON THOMAS, HARRYSSON, RALPH, HERMANSSON, LEIF, JOHANSSON, THOMAS
Applicants: HARRYSSON RALPH, HERMANSSON LEIF, JOHANSSON THOMAS, LUCOCER AB, HARRYSSON, RALPH, HERMANSSON, LEIF, JOHANSSON, THOMAS, LUCOCER AKTIEBOLAG
Application year: 1994
Main classification: A61F2
IPC codes: A61C8, A61F2, A61L27